The goal of this proposal is to provide insights into the molecular mechanism of innate immunity. Forward and reverse genetic studies have led to the identification of C. elegans signaling pathways that are required for defense response in both nematodes and mammals, suggesting that despite the vast evolutionary gulf between nematodes and mammals, some of the underlying mechanisms of defense response may be similar. We have used a set of C. elegans mutants hypersusceptible to pathogens to define a defense response pathway that involves the CED-3 programmed cell death pathway and the C. elegans homolog of the mammalian p38 mitogen-activated protein kinase (MAPK) encoded by the pmk-1 gene. Also, lipopolysaccharide (LPS) was found to act as a pathogen-associated molecular pattern (PAMP) that triggers a CED/MAPK-dependent programmed cell death in C. elegans. Our preliminary results indicate that ced-1 worms are hypersusceptible to S. enterica, ced-1, that is required for efficient clearance of apoptotic cells and cooperates with ced-3 to promote cell death, encodes a homolog of the mammalian CD91 receptor which has been involved in the elicitation of innate and adaptive immune responses by recognizing heat- shock proteins, including HSP90. In addition, to establish parallels between innate immunity in mammals and nematodes, we have studied the expression profile of mice exposed to LPS to identify PAMP- responsive genes and the role of the C. elegans homologs in defense response was analyzed. Three C. elegans mutants in LPS-responsive genes were tested for their susceptibility to bacterial pathogens and two of them were found to be deficient in defense response. One of the mutants is daf-21(p673), which has a missense mutation in the C. elegans homolog of mammalian hsp90. First, we propose to continue our preliminary studies involving ced genes to determine the mechanisms by which these genes trigger defense response and the relationship between cell death and susceptibility to pathogens. We will also further characterize the role of DAF-21/HSP90 in defense-response and dissect its interaction with the CED-1/CD91 pathway. Finally, we propose to use bioinformatics and functional and comparative genomics to identify and characterize defense-related genes required for innate immunity in both nematodes and mammals.